An RF switch is an electronic "on-off" switch for high frequency RF signals. The switch permits an RF signal applied to an input port to normally propagate to a switch output port, when "on", and, alternately, block passage of such high frequency RF signals, when turned "off". The switch is turned on or off depending upon the polarity of a DC control voltage applied to the switch's control input. As is known such kinds of switches are typically used in transmit/receive and interrogation systems.
RF switches come in a variety of configurations. The basic one is a single pole single throw (SPST), containing a single RF input and RF output. Others come in single pole multiple throw (SPMT) configurations. A typical RF switch contains two ports or outputs, which are separately controlled, that are fed from a single input port, and is referred to as a single pole double throw (SPDT) configuration. In that switch, one of the output ports may be "on" while itched "off", and vice-versa. Typically the RF switch accomplishes such electronic switching with RF switching diodes, also known as PIN diodes, a principal component element of the RF switch's electronic circuitry.
The term RF switching diode or switching diode as used in this specification refers to a PIN diode, which is believed to be the only diode used at present to switch RF signals as described. However, it is understood that the term is a functional one and has a more generic meaning and intent. The term is intended to encompass any other diode or diode equivalent device as well that in the future may be discovered which, though accomplishing the same and/or additional functions and characteristics to that accomplished with the PIN diode, becomes known by a name different than PIN. It also can encompass the surface mounted impedance elements, such as capacitors.
As is known to those skilled in the art, the PIN diode is a two terminal semiconductor device. As in any diode, it conducts current in only one direction, from the positive polarity terminal or anode to the negative polarity terminal or cathode, as variously termed. With a positive voltage on its anode, the diode is forward biased and will conduct current. If biased to conduct current, that is, if forward biased, the diode offers very little or essentially no resistance to current flow. It is in its "on" state. If oppositely biased, that is, is back biased, the diode offers an infinitely high resistance and forms an essentially open circuit through which electrical current cannot normally pass. It is in its "off" state.
The diode requires a finite time to change from the one state to the other with a change in voltage. That characteristic is referred to as the diode's "transition time". In order to change the diode's conducting state from one state to the other, the new voltage biasing the diode to the opposite conductive state must be applied to the diode for the diode's minimum transition time.
Any alternating current (AC) signal, such as an RF voltage, that is superimposed upon the back biasing DC control voltage does not change the PIN diode's non-conducting state, so long as the frequency of that signal is sufficiently high, such that the duration of the voltage swings or peaks in the signal is insufficient to satisfy the minimum required time to transition the diode from an "off" state to an "on" state. However, by changing the polarity of the DC,control voltage so as to forward bias the diode, the diode changes its conducting state and conducts current, including the AC signal. When thus forward biased, as was true in the back biased condition, any alternating current signal, AC, of a sufficiently high frequency that is superimposed upon the forward biasing DC control voltage does not change the diode's conducting state.
There is a limit to such superimposed signals. If the AC is high enough in voltage it could exceed the diode's breakdown voltage and destroy the diode. However, to avoid that, the PIN diode is selected such that the anticipated maximum level of the superimposed AC signal should never exceed the selected diode's breakdown voltage.
Thus a unique characteristic of the PIN diode is that, when forward biased, it conducts the RF signal even though the peak voltage levels of that RF signal exceed the level of that forward bias. By design the RF frequencies employed in the electronic systems using the diode as an RF switch are sufficiently high in frequency so as not to alter the conductive state in which the diode is placed by the DC control voltage biasing the diode. Additional technical information on the PIN diode and its application in RF switches, if of interest, may be found in the following publications: "Pin Diode Operation and Design Trade-Offs, Sahjani & White, Applied Microwave, Spring 1991, pp 68-78; "Broadbanding the Shunt PIN Diode SPDT Switch, Application Note 957-1, Hewlett Packard Co. Febrero. 1990 (2 pages); "Design with PIN Diodes", Hiller, bulletin, AG312, MA-COM Semiconductor Products Div. 1986 (18 pages); "How to Specify PIN Diode Switches", (pp 26-31) American Microwave Corp., Application Note 1-0690, June 1990, and "Simplifying the specs of PIN-diode switches", Algeri & Hicks, Microwaves & RF, July 1986 (pp 83-91).
In its construction, a shunt RF switch takes advantage of the PIN diode's electrical characteristics. Speaking generally, with a PIN diode placed in shunt of an RF transmission line and back biased by the control voltage, the diode appears as an open circuit; permitting applied RF to propagate along the transmission line to the output. When, however, the diode is forward biased and thereby conducts current, it offers a very low impedance path to the RF, and shunts the RF from the transmission line through the diode to electrical ground potential, bypassing the transmission line output or, as variously termed, output port. The RF, therefore, cannot pass further along the transmission line. The RF switch's output port, being bypassed, is effectively isolated from the switch's input.
With RF present or not at the output port in dependence on the control voltage applied to the PIN diode, the foregoing shunting device effectively forms a single pole single throw (SPST) RF switch. As is known, a practical shunt RF switch typically includes additional components to provide DC isolation between the switch's input and output ports and one or more shunt switching diodes to implement the foregoing shunting action.
The RF switch is packaged in a single mechanical assembly that holds the components in a predefined physical relationship or in a larger package in common with other electronic devices that serve together as components of a more extensive electronic circuit. That is, the switch circuit is installed on a printed circuit board or the like, wherein the components are electrically inter-connected in the appropriate electrical circuit via electrical conductors, microstrip traces, formed on the circuit board; the components and supporting circuit board mechanically define a unitary assembly, the package.
PIN diodes and other diodes are commercially available, unpackaged, as a die, which must be wire bonded by the purchaser to connect to other circuit elements, and are available in a variety of surface mount cases or, as alternately termed, surface mount packages. One such diode package is mechanically configured in a generally rectangular or square shape overall for convenient mounting to the surface of the circuit board. The diode's anode and cathode terminals are metal pieces that are somewhat rectangular in shape, are of a small thickness, and have a flat outer surface. They are slightly larger in length and width dimension that the insulated portion of the body. Those terminals are arranged at the front and rear ends of the diode and are suitable for soldering to a metal trace on a printed circuit board. PIN Diodes of such configuration are also referred to variously as surface mount diodes or packaged surface mount PIN diodes or packaged PIN diodes, and the terms should be regarded as synonymous. It thus should be understood that when reference is made in this specification to any of a packaged PIN diode or packaged surface mount PIN diode or simply to a PIN diode, the reference is to the afore described packaged surface mount structure.
The terminal's flat end surface permits the electrical current that passes through the diode to be distributed over the widest possible area when the terminal is soldered to a metal trace of like dimension. When, however, the diode is placed on its side, the electrical current must pass through the smaller area available through the side edge of the terminal, resulting in a higher current density than with the former connection.
Generaly, parasitic elements introduced by component mounting hinders the ability of achieving high performance levels from a surface mount diode in high frequency RF applications. And it is necessary to minimize any additional parasitics, such as inductance and resistance, introduced by mounting the component to the circuit.
At low RF frequencies, little concern is given to the electronic characteristics of the electrical lead and its connection to the solder pad, other than ensuring a firm solder connection. A connection to the solder pad possesses an inherent inductance, resistance and capacitance, but the resultant parasitic impedance at low frequencies is quite low and does not significantly affect the operation of low frequency circuits. At very high RF frequencies, such as those contemplated by the present invention, the pad's electronic characteristics become very significant, particularly in connection with the operation of switching diodes, the subject of the present application. The inductance, resistance and capacitance of the solder pad at the high frequencies can produce a significant impedance.
In the described shunt RF switch, the output or port is "turned off" by activating the diode, which places a very low impedance path from the microstrip trace to electrical ground that bypasses and effectively isolates the output port from the input. On/off port isolation is reduced, however, if any parasitics increase that impedance. When impedance in series with the diode is increased, the diode no longer completely shunts the output port. Some portion of the signal then appears as a voltage drop across the impedance and also appears at the output port. Thus the ratio of the output port RF level, when the port is "off", to the corresponding input RF level, the measure of port isolation, increases.
In RF applications a widely used and produced printed circuit board is referred to herein as a "stand-alone" circuit board. It typically contains plated-on metal traces defining a circuit pattern, means for mounting electrical components for connection within the circuit pattern and a plated-on thin metal layer on the underside to serve as the electrical ground. On this packaging, the surface mount switching diode is mounted lying down, that is, lying on its side against the upper surface of the board, with the narrow side edge of one of its end terminals soldered in place to a microstrip metal trace and another narrow side edge of the opposite end terminal soldered to a nearby ground pad.
In that prior stand alone circuit board diode mounting configuration, the electric current conducted through the diode is concentrated near the terminal's side edges. The only path for the current from the diode is through the side edge of the end terminal to the solder pad on the metal trace. The current through the upper edge of that end terminal is consequently lower than it should be due to the resultant pad parasitics. Because the area of this side edge is much less than that of the end of the terminal, it presents a smaller path for current flow than does the entire terminal end. A smaller path effectively creates greater resistance to current flow; and the resultant effect is an increase in parasitic elements, the resistance, capacitance and inductance. The small area at the side edge routes all diode current. As a consequence, the current density is the greatest at this point, and that prevents optimal current flow. Thus, the foregoing stand-alone circuit board and "lying down" diode mounting configuration could not provide reasonable isolation for high frequency shunt RF switches.
To minimize those parasitics in the RF switch, packaged PIN diodes have heretofore been mounted to thick metal plate mounted printed circuit boards using a "tented diode" configuration. In this printed circuit board, a thick metal backing plate underlies and supports the printed circuit board; and the metal plate serves as an electrical ground. The circuit board contains a cut-out opening through the circuit board's insulating material, exposing a portion of the underlying thick metal plate. The surface mount PIN diode, being a short rectanguloid figure in shape, containing square shaped metal ends that serve as the diodes electrical terminals, is mounted on end, upstanding, to the thick metal plate, perpendicular to the circuit board. One of the diode's end terminals extends through that cut-out opening to full end contact with the underlying thick metal backing plate and is soldered in place thereto, leaving the diode's remaining end terminal elevated above the surface of the printed circuit board.
To complete the diode's mounting, a metal ribbon, which serves as an electrical lead to the diode, is placed across the cut-out region, over and in electrical contact with the diode's upstanding end terminal; and the ends of that metal ribbon are connected to appropriate metal traces on the circuit board on opposite sides of the diode. Those metal traces provide the connection to other circuit elements on the circuit board. In appearance the foregoing ribbon resembles a familiar pup tent and, hence, the diode is said to be "tented". The present invention also involves tenting the switching diode.
Ribbon tenting of the packaged diode effectively provides a secondary path for the pad current and a larger conducting area, which lowers parasitic induced impedance and allows near uniform pad current flow. Electrical current through the diode is uniformly distributed through the face of the electrical terminal and adjacent solder pad on the circuit board. The increase in conducting area produces a decrease in diode pad parasitics. That phenomenon occurs on both sides of the diode since the opposite pad makes full contact with the microstrip trace. In one practical embodiment, tests reveal a 13.7 dB increase in isolation going from a side mounted architecture to the tented configuration.
Despite the disadvantage of increased weight and expense of the thick metal backing plate and a cut-out on the printed circuit board such mounting configuration offered the only viable choice for shunt RF switches with discrete packaged parts. As a consequence the circuit designer is given a single choice of packaging, but that choice was one that satisfied all of the designer's needs. Despite long-standing engineering practice of minimizing or reducing weight in airborne and spacecraft electronics systems and seemingly natural goals of lowering manufacturing costs of electronic equipments, as would naturally accrue from use of a stand-alone circuit board in shunt RF switch applications, until the present invention, a stand-alone packaging system remained impractical for shunt RF switch applications.
The present invention resurrects the stand-alone printed circuit board as part of the mounting structure for a packaged PIN diode in a shunt RF switch, yet permits optimal current flow and results in lower parasitics than the prior shunned stand-alone circuit board mounting structure. Effectively the new mounting configuration emulates the electrical characteristics of the thick metal backing plate printed circuit board diode mounting configuration previously described; and gives the RF switch designer an alternative choice of mounting.
Accordingly, an object of the present invention is to provide a new mounting structure for a packaged surface mount PIN diode.
A further object of the invention is to improve on/off port isolation of a surface mount shunt semiconductor RF switch packaged on a stand-alone printed circuit board.
A still further object of the invention is to provide a new mounting structure for a packaged PIN diode using a stand-alone printed circuit board in a combination that electrically emulates the prior plate mounted printed circuit board package for that diode.
And an additional object of the invention is to reduce the cost of manufacturing shunt RF switches while maintaining existing performance capability.